“Masters of Sex” returns Sunday for a second season to explore the relationship that led to a revolution.

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Michael Sheen and Lizzy Caplan in season 2 of "Masters of Sex" on Showtime

The scene arrived with all the trappings of a run-of-the-mill romantic comedy ending: The guy, down and out, stands in a pouring rain on his on-again-off-again crush’s doorstep, shifting and stammering as he makes a declaration that just falls short of love: “I finally realized there is one thing I can’t live without – it’s you.”

In the movies, the credits would roll over a happy tune and we’d be left to imagine the couple’s happily-ever-after existence. But on “Masters of Sex,” the Season 1 capper only set the stage for the next complicated phase in the odd coupling of Dr. William Masters and former singer Virginia Johnson, whose 1950s groundbreaking study of human sexuality lives eons from contemporary rom-com fodder.

The show, which packs a deceptive emotional punch that speaks less to the mysteries of love than attraction, returns to Showtime for a second season Sunday with viewers drawn by hopes for another master class in TV drama.

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"Masters of Sex" premiered last September, seemingly rising out of nowhere to tide over "Mad Men" fans between seasons with its similar serious retro sensibility and a (mostly) satisfying slow pace.

Comparison are both unfair and apt: 1960s-set “Mad Men” helped pave the way for sophisticated TV looks at the not-so-simple lives of past American generations during different post-war periods. But “Masters of Sex” quickly distinguished itself, like Masters and Johnson did, by smartly tackling taboos and conflicts that, on closer inspection, don’t seem all that dated.

Johnson struggles to balance her career, motherhood and life as a sexually liberated woman. Masters grapples with a medical establishment unwilling to put aside preconceptions, politics and notions of morality to accept findings based in science. The doctor, who harbors some serious mommy and daddy issues, also battles repression – but not near as much as his married, closeted boss, who sadly is driven to aversion therapy to “cure” himself (The subplot, featuring Beau Bridges and Allison Janney, proved a powerful Season 1 highlight).

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Masters and Johnson find themselves searching for answers, at great personal expenses (including the crumbling of his marriage) – and in strange places, conducting sexual experiments everywhere from a St. Louis hospital to a brothel.

Those scenes, however, aren’t played for guffaws – they’re products of Masters and Johnson’s passion, which has extended from their work to one another.

Michael Sheen’s controlled-yet-restless turn as Masters and Lizzy Caplan’s driven-yet-conflicted portrayal of Johnson (she's up for a lead actress drama Emmy for her portrayal) infuses the sense of unease that permeates a show about a couple’s hunt for something beyond love and the key to sexual satisfaction.

They only clear thing is that Masters and Johnson need each other: “Masters of Sex” is about a relationship as much as the start of a revolution. Check out a preview of Season 2 below: